A remarkable feature of the nervous system is its capacity to rapidly process sensory information, compare that with previous experiences, and then select and initiate an appropriate behavioral response. This process has salient implications for several cognitive functions, including learning and decision-making. Decision- making defines the selection of one out of several potential behavioral responses to a given situation, and deficits in decision-making have been linked to drug addiction and cognitive disorders, including anxiety disorders and schizophrenia. Despite its biological and clinical relevance, our understanding of the genetic mechanisms and the neural circuits underlying decision-making is limited. Zebrafish larvae show a remarkable degree of behavioral plasticity, and we previously established an automated high-throughput system to quantify behavioral plasticity of the acoustic startle response, including habituation learning. We have modified this system to assay simple decision-making, based on our earlier discovery that in response to acoustic stimuli zebrafish respond with one of two stereotyped motor behaviors: Short-Latency C-bends (SLC) representing a fast escape behavior displayed e.g. when encountering a predator, or Long-Latency C-bends (LLC) which represent more an exploratory re-orientation behavior. We used four hallmark features of more complex decision making and confirmed that our assay fulfills all four characteristics, strongly suggesting that our assay indeed measures simple decision making. To identify genes critical for simple decision making, we performed a forward genetic screen that identified 10 mutants with specific defects SLC versus LLC bias, representing the first vertebrate mutants specifically isolated based solely on behavioral choice deficits. Through whole genome sequencing of one of these mutants we identified a missense mutation in the calcium sensing receptor (CaSR) gene. CaSR encodes a vertebrate specific G- protein-coupled receptor (GPCR) sensitive to extracellular calcium, and its role in regulating plasma serum Ca2+ is well understood. Mutations in human CaSR have also been linked to idiopathic epilepsy, and while CaSR is expressed in neurons its role in the CNS is not well established. Finally, we established GCaMP6 based brain imaging to monitors neuronal activity during behavioral choice selection in behaving animals. Here, we propose to full advantage of the zebrafish system to map hindbrain neurons selective and functionally required for SLC versus LLC startle (Aim 1). We will then apply this knowledge in combination with transgenic analyses to determine how the G-protein-coupled receptor CaSR regulates SLC versus LLC choice (Aim 2a), and we will clone 9 additional SLC versus LLC startle bias mutants via whole genome sequencing to define additional molecular entry points into the SLC versus LLC behavioral choice (Aim 2b). Combined, the proposed experiments will provide unprecedented insights into the neural circuitry and molecular mechanisms critical for simple decision-making in vivo.